This invention relates generally to the field of electrical power generation, and more particularly to the generation of electrical power using an electrochemical fuel cell.
A fuel cell is an energy conversion device that efficiently converts stored chemical energy into electrical energy. Conventional fuel cells generally operate by combining hydrogen with oxygen to generate direct current electrical power. The overall chemical reaction for this process is described by Equation 1.
2H2+O2xe2x86x922H2Oxe2x80x83xe2x80x83Equation 1. 
In order to generate enough power to be usable for practical applications, multiple fuel cells are combined electrically in series. In this stacked configuration, the individual fuel cells are connected one after another, similar to the cells of a conventional voltaic battery.
At its fundamental level, the individual fuel cell contains an electrode at which oxygen is reduced (the cathode) and another electrode at which fuel is oxidized (the anode). Fuel cell electrodes are generally of the gas diffusion type and are made of an electrically-conducting support material, an active catalytic layer, and an electrolyte.
Chemical pore-formers have been used to control the porosity of these layers. These pore-formers, or porophoric agents, are added as powders or crystals to the compositions of the various layers. The pore-formers eventually decompose in the gas phase or are dissolved in solution in post-fabrication steps. Conventional pore formers include powdered ammonium bicarbonate, ammonium chloride, and urea, which are either lost in the gas phase or via dissolution. Such particles are generally coarse and operate like yeast in dough, forming a sponge-like structure with rather coarse porosity. These pore-formers are too large to be compatible with the thin active layers that can be prepared by methods such as paint- or ink-like application or rolling (calendering).
The membrane electrode assembly (MEA), which is composed of the anode, cathode and electrolyte membrane, is generally sandwiched between gas flow-fields. These flow fields allow the reactant gases (separate streams of H2 and O2) to contact the MEA. Conventional flow-fields are formed by pore-free grooved graphite plates, wherein the reactant gases flow through a serpentine-shaped groove. A drawback to this type of flow field is that it requires heavy tie rods and end-plates to compress the flow-fields and MEAs together to maintain electrical contact.
In a fuel cell stack, the individual fuel cells are connected to each other by bipolar plates. The bipolar plates provide electrical contact between the cells and may also be involved in cooling the stack. Once again, the traditional approach is to maintain contact between the cells and the bipolar plates by applying pressure to the stack using end-plates squeezed together by heavy tie rods.
Porous metals are an attractive alternative to heavy graphite flow fields. Porous metals that have been used as flow field material include porous copper, porous nickel, porous aluminum, porous titanium, and porous aluminum-titanium alloys (U.S. Pat. Nos. 6,022,634 and 6,146,780). The fuel cells shown in U.S. Pat. No. 6,022,634 use porous metal flow fields and collapsed porous metal current collectors. These components are pressed together between endplates and still require tie rods or some means of applying external pressure to the cells to maintain electrical contact between the components. Flow-fields shown in U.S. Pat. No. 6,146,780 are made of metal foams that are spot welded to gas impermeable bipolar separator plates.
The polymer electrolyte membrane (PEM) material generally used in polymer electrolyte membrane fuel cells (PEMFCs) is generally composed of a linear, branched-chain perfluorinated polyether polymer with a non-crosslinked structure that has terminal sulfonic acid endgroups. An example of such material is Nafion(copyright) (Du Pont de Nemours and Company, Wilmington, Del.). Nafion(copyright) requires a substantial amount of water (typically 10-20 water molecules per sulfonic acid group) to give adequate proton conductivity. The high water requirements are due to the volume occupied by the hydrophobic fluorinated sulfonic acid polymer chain. Proton conduction only takes place down self-organizing hydrophilic channels or micelles which occupy only a small portion of the total superficial area of a PEM electrolyte film, reducing the corresponding specific conductivity compared with the local value in the channels. The hydrophobicity of the polymer chain also limits the local amount of water associated with the sulfonic acid. This amount of water increases rapidly as the equivalent weight or the ratio of the molecular weight of polymer chain to sulfonic acid becomes less, and it falls with increasing temperature. Conductivity is highest when liquid water is in contact with the membrane at any given temperature. For this reason, developers have generally supplied PEMFCs with the reactants (hydrogen and air) humidified to at least the cell operating temperature, so that product water is formed in the liquid state.
Since the system must generally operate below the local boiling point of water, excess water used for humidification, plus the product water from the reaction, collects in the cathode gas flow channels. Means must therefore be provided to continuously remove it. The fact that the PEMFC produces liquid water under normal circumstances is a major operational flaw and considerable ingenuity is required to deal with it, especially in larger cells.
In the General Electric Company fuel cell used in the Gemini space missions starting in 1965, water was required to be removed in a microgravity environment. Water management was accomplished by providing a wicking material in the cathode flow channels of each cell. The exit end of the wick in each individual cell communicated over the active width of the cell with a porous water separator plate arranged in parallel with the cells in the stack. A differential oxygen pressure drove the water through this separator plate to a product water accumulator for storage as drinking water and for recycle to the entry side of each wick to maintain conductivity (Appleby et al., Energy 11: 137 (1986); Liebhavsky et al., Fuel Cells and Fuel Batteries, Academic Press, 587 (1964)).
In the Gemini cell, the membrane was not fluorinated for stability and operated at close to ambient temperature and at low current density for high efficiency. The non-fluorinated membrane was prone to hot-spots with gas-crossover, and was replaced by Nafion(copyright) as soon as it became available in the late 1960s. In the Ballard stack design (Prater et al., J. Power Sources, 61: 105 (1996), U.S. Pat. Nos. 5,521,018, 5,527,363, and 5,547,776), the water is forced out of the cathode flow channels by applying a large pressure differential between the inlet and exit side. This requires the use of very long, serpentine flow channels, each with a length many times the cell width (U.S. Pat. Nos. 4,988,583 and 5,108,849). While Ballard has looked at other systems for removing water, a vapor phase feedback loop at the anode (after back-diffusion from the cathode channels) (U.S. Pat. No. 5,441,819), the serpentine channel design is still retained. This design means that the stack will only operate under pressurized conditions, with a minimum operating pressure between 2 and 3 atmospheres absolute (atma) of air. Pressurized operation requires either a stack with heavy filter-press components or a pressure vessel surrounding the stack. Both of these reduce the flexibility of stack design, and necessitate the use of liquid cooling system with an external radiator. The reactants are prehumidified to cell operating temperature by passing them through membrane-humidification cells which may or may not be arranged en bloc with the electrochemical stack. The water circulation in the humidification cells is deionized cooling water exiting the cooling plates (generally, one every 4 or 5 active cells) in the electrochemical stack.
In the International Fuel Cells stack design (U.S. Pat. Nos. 4,769,297 and 5,503,944), a graphite wicking plate with controlled porosity is used on the cathode side. The graphite plate has deionized water cooling flow channels on its reverse side, which contact a similar porous graphite wicking plate on the anode side of the next cell. Thus, there is one cooling plate per cell. The anode side of each cell is pressurized to xe2x88x920.075 to xe2x88x920.15 atmospheres gauge (atmg) compared to the cooling water stream, and the cathode side is correspondingly at +0.075 to +0.15 atmg. This means that there is a net flow of pure water from the cooling stream to the anode for humidification, and a corresponding flow of this and product water from the cathode channels to the cooling stream. All of the above approaches require controlled pressure differentials that are unsuitable for use in small lightweight stacks. They also use a water-cooling subsystem, which requires a heat exchanger, pumps, and controls, which add weight, as do the elements required for humidification. While such humidification is active, it proceeds automatically in each cell, and may be called internal humidification to distinguish it from external humidification via the reactant gases before they are led into the cell.
In true internal- or self-humidification, a portion of the water produced in the fuel cell reaction is reabsorbed by the membrane electrolyte keeping the latter moist during fuel cell operation (U.S. Pat. Nos. 5,242,764 and 5,318,863; Dhar, J. Electroanal. Chem., 357: 237 (1993)). This requires back-diffusion of water from the cathode to the anode side, hence a thin PEM film. In addition, it requires the use of rather short flow channels, so that efficient back-diffusion of water can occur, preferably in a counter-flow system or one operating dead-headed on hydrogen. It also requires careful control of oxygen utilization, hence air-flow rate as a function of current density, as well as that of cell temperature. For example, if oxygen utilization in dry air cathode reactant is 50%, the cathode exit gas is humidified to 0.182 atm, or 138 torr at 1 atma total pressure. This sets an upper limit of cell operating temperature of about 59xc2x0 C. at the cell exit, since this must not exceed the dew point of the reactant gases at any point in the cell. The cathode exit should also be the hottest part of the cell so if the system is air-cooled by a separate air stream, cooling air and process air should be co-flow. In practice, the maximum average cell temperature should be in the 50-55xc2x0 C. range to ensure that the anode and cathode inlets, which operate under the lowest dew point conditions, function satisfactorily.
One aspect of the invention is a method of making a gas diffusion electrode comprising the steps of forming an electrode on a substrate by applying a mixture comprising a polymer electrolyte, an electrocatalyst, and a nanosized pore-former to the substrate; and treating the electrode to remove the nanosized pore-former. According to one embodiment the pore-former comprises fumed silica.
A further aspect of the invention is a method for making a membrane electrode assembly comprising the steps of forming an electrode on a substrate by applying a mixture comprising a polymer electrolyte, an electrocatalyst, and a nanosized pore-former to the substrate; attaching the electrode to a membrane to form a membrane electrode assembly; and treating the electrode to remove the pore-former.
A still further aspect is a method for making a gas permeable layer for use in a gas diffusion electrode comprising the steps of applying to a composition a nanosized pore-former; and treating the composition to remove the nanosized pore-former. A further aspect of the invention is an electrode comprising a polymer electrolyte, an electrocatalyst, and nanosized pores.
Another aspect of the invention is an electronically conducting fuel cell component comprising a porous metal flow field, an intermediate layer bonded directly to the porous metal flow field, and an electrode bonded directly to the intermediate layer. A still further aspect of the present invention is a method for making an electronically conducting fuel cell component comprising the steps of directly bonding an electrically conducting intermediate layer to a porous flow field, and directly bonding an electrode to the intermediate layer.
A further aspect of the invention is a fuel cell comprising first and second monolithic electrically conducting flow field-bipolar plate assemblies arranged essentially parallel to each other such that an inside surface of the first flow field-bipolar plate assembly is facing an inside surface of the second flow field-bipolar plate assembly, wherein the flow field-bipolar plate assemblies are electrically and mechanically connected by intervening layers. The intervening layers may comprise a first electrically conducting intermediate layer bonded directly to the inside surface of the first flow field-bipolar plate assembly, a second electrically conducting intermediate layer bonded directly to the inside surface of the second flow field-bipolar plate assembly, a first electrode bonded directly to the inside surface of the first electrically conducting intermediate layer, a second electrode bonded directly to the inside surface of the second electrically conducting intermediate layer, and a polymer electrolyte membrane between and bonded directly to both of the electrodes.
A still further aspect of the present is an electrochemical fuel cell stack comprising two electrically conducting end-plates and a plurality of electrochemical fuel cells disposed between the endplates. The electrochemical fuel cells may comprise first and second monolithic electrically conducting flow field-bipolar plate assemblies arranged essentially parallel to each other such that an inside surface of the first flow field-bipolar plate assembly is facing an inside surface of the second flow field-bipolar plate assembly, wherein the flow field-bipolar plate assemblies are electrically and mechanically connected by intervening layers. The intervening layers may comprise a first electrically conducting intermediate layer bonded directly to the inside surface of the first flow field-bipolar plate assembly, a second electrically conducting intermediate layer bonded directly to the inside surface of the second flow field-bipolar plate assembly, a first electrode bonded directly to the inside surface of the first electrically conducting intermediate layer, a second electrode bonded directly to the inside surface of the second electrically conducting intermediate layer, and a polymer electrolyte membrane between and bonded directly to both of the electrodes.
A further aspect of the invention is a method of making a fuel cell stack comprising disposing between two electrically conducting endplates a plurality of electrochemical fuel cells, wherein the electrochemical fuel cells comprise first and second monolithic electrically conducting flow field-bipolar plate assemblies arranged essentially parallel to each other such that an inside surface of the first flow field-bipolar plate assembly is facing an inside surface of the second flow field-bipolar plate assembly, and wherein the flow field-bipolar plate assemblies are electrically and mechanically connected by intervening layers. The intervening layers may comprise a first electrically conducting intermediate layer bonded directly to the inside surface of the first flow field-bipolar plate assembly, a second electrically conducting intermediate layer bonded directly to the inside surface of the second flow field-bipolar plate assembly, a first electrode bonded directly to the inside surface of the first electrically conducting intermediate layer, a second electrode bonded directly to the inside surface of the second electrically conducting intermediate layer, and a polymer electrolyte membrane disposed between and bonded directly to both of the electrodes.
A still further aspect of the invention is method of generating electrical power comprising supplying hydrogen and oxygen to an electrochemical fuel cell stack, wherein the electrochemical fuel cell stack comprises two electrically conducting end-plates and a plurality of electrochemical fuel cells disposed between the endplates; wherein the electrochemical fuel cells comprise first and second monolithic electrically conducting flow field-bipolar plate assemblies arranged essentially parallel to each other such that an inside surface of the first flow field-bipolar plate assembly is facing an inside surface of the second flow field-bipolar plate assembly, and wherein the flow field-bipolar plates assemblies are electrically and mechanically connected by intervening layers. The intervening layers may comprise a first electrically conducting intermediate layer bonded directly to the inside surface of the first flow field-bipolar plate assembly, a second electrically conducting intermediate layer bonded directly to the inside surface of the second flow field-bipolar plate assembly, a first electrode bonded directly to the inside surface of the first electrically conducting intermediate layer, a second electrode bonded directly to the inside surface of the second electrically conducting intermediate layer, and a polymer electrolyte membrane between and bonded directly to both of the electrodes.
Yet a further aspect of the invention is an air cooled condenser for use with a fuel cell stack, the condenser comprising a three-dimensionally reticulated porous metal condensing element and a three-dimensionally reticulated porous metal cooling element, wherein the three-dimensionally reticulated porous metal condensing element is disposed between two gas impermeable barriers by continuous metallurgical bonds, and wherein the three dimensionally reticulated porous metal cooling element is disposed between and bonded directly to two other gas impermeable barriers.
A further aspect of the present invention is an evaporatively cooled internally humidified fuel cell stack comprising a plurality of fuel cells and an air cooled condenser in fluid communication with the fuel cells, wherein the condenser comprises a plurality of three-dimensionally reticulated porous metal condensing elements and a plurality of three-dimensionally reticulated porous metal cooling elements, wherein the three-dimensionally reticulated porous metal condensing elements are disposed between and bonded to two gas impermeable barriers by continuous metallurgical bonds, and wherein the three-dimensionally reticulated porous metal cooling elements are disposed between and bonded directly to two other gas impermeable barriers.
A still further aspect of the invention is a method of cooling an electrochemical fuel cell comprising placing the electrochemical fuel cell in fluid communication with an air cooled condenser wherein the air cooled condenser comprises a plurality of three-dimensionally reticulated porous metal condensing elements and a plurality of three-dimensionally reticulated porous metal cooling elements, wherein the three-dimensionally reticulated porous metal condensing elements are disposed between and bonded to two gas impermeable barriers by continuous metallurgical bonds, and wherein the three-dimensionally reticulated porous metal cooling elements are disposed between and bonded directly to two other gas impermeable barriers.
A still further aspect of the invention is a flow field-bipolar plate assembly for an electrochemical cell, comprising a first and second three-dimensional reticulated porous metal flow-fields bonded directly to opposite sides of an electrically conducting gas impermeable barrier by continuous metallurgical bonds.
A further aspect of the invention is a method of delivering a gas to a fuel cell electrode comprising delivering the gas to a porous metal flow field-bipolar plate assembly wherein the porous metal flow field-bipolar plate assembly comprises an electrically conducting gas barrier and a three-dimensionally reticulated porous metal flow field bonded to one side of the electrically conducting gas barrier by a continuous metallurgical bond; wherein the gas contacts the three-dimensionally reticulated porous metal flow field and diffuses into contact with an electrode that is in gas communication with the three-dimensionally reticulated porous metal flow-field.